This invention relates to a stereophonic system for AM broadcast transmitters and receivers. Specifically, apparatus is provided which is compatible with present AM modulated transmitting and receiving apparatus for transmitting two channels of information.
Two channel transmission incorporating FM modulation techniques are well known and widely used at frequencies above 50 MHz. It has been proposed by numerous authors to transmit two channels of information by means of amplitude modulation on a low frequency wave. The AM stations currently operating in the region of 550 KHz to 1600 KHz are not operated as stereo transmitting systems but remain as transmitters of monophonic information only. Therefore, it would be desirable to upgrade the quality of low frequency (550 KHz to 1600 KHz) amplitude modulated signals by including a second channel of information which could be received and demodulated to provide two channels of information for stereophonic reception.
Stereophonic systems for low frequency AM modulated transmitters must be compatible with present day transmitters and receivers of low frequency amplitude modulated signals. This is necessary in order to accommodate the millions of receivers in current use with new proposed stereophonic broadcasts.
A number of two channel systems have been proposed in the past which are compatible with monophonic transmitting and receiving equipment. One such system is described in I.E.E.E. Transactions on Broadcasting, Volume BC-17, No. 2, June 1971, pages 50-55. The system described in this particular paper transmits two signals comprising an L-R signal and an L+R signal. The L-R signal is phase shifted and then applied to a balanced modulator. A carrier signal is supplied to the balanced modulator and a double sideband, suppressed carrier signal is produced. The double sideband, suppressed carrier signal is added to a carrier signal which has been shifted 90 degrees. This composite signal comprising a carrier shifted at 90 degrees and a double sideband suppressed carrier signal is used as the basis for deriving an RF signal to be modulated with still another source of information, L+R. The double sideband signal plus phase shifted carrier is frequency multiplied to a suitable carrier frequency for transmission.
The frequency multiplied signal is AM modulated with a second source of signal, L+R, which is also phase shifted. The resulting composite signal includes a first sideband containing the left signal and a second sideband containing the right signal.
The transmitted two channel signal may be received by tuning two separate receivers to the first sideband and to the second sideband. By tuning in this manner, the L and R signals are recovered.
The system, however, does not achieve a high degree of isolation between channels, and cross talk is evident. The I.F. filter bandwidth and skirt slope is such that a portion of the upper sideband would necessarily enter the receiver passband which was tuned to the lower sideband. To achieve better isolation between information channels, the I.F. filter bandwidth must have very sharp skirts and a high stop band attenuation level.
Another system which has been described for transmitting stereophonic AM signals comprises an FM signal for carrying one signal channel, and a true AM modulation of the resulting FM modulated signal by the remaining signal channel. The modulated FM is derived by frequency modulating a carrier signal with pre-emphasized audio signal. A pre-emphasis network imparts a higher level to higher frequency audio signals than to lower frequency audio signals. The transfer function for the pre-emphasis network is directly proportional to the frequency of an input audio signal over the effective pre-emphasis bandwidth. In actual practice, the pre-emphasis network may be realized by operating an R-C high pass filter in the skirt region where the frequency response of the filter increases linearly. This give a positively increasing slope to the amplitude-frequency response of an audio signal which is used to modulate an FM modulator. The modulated signal has the characteristic of a PM signal rather than FM over the limited region of effectual pre-emphasis.
The resulting frequency modulated signal is supplied to an AM full carrier double sideband transmitter where it is modulated with a second audio signal. The composite FM/AM signal appears over a limited audio frequency range as a phase modulated signal with AM modulation impressed upon it, and as an FM signal with AM modulation over a limited low audio frequency range.
A shortcoming with the pre-emphasized FM/AM system has been experienced in that the pre-emphasis is obtained over a limited region of the input audio frequency spectrum. Where pre-emphasis is not effective, wide band FM occurs which is a potential source of distortion. The wide band FM resulting from limited pre-emphasis tends to cause FM-to-AM conversion in the tuned circuitry of the receiver. The conversion results from slope detection of the FM signals produced by the wide deviation of the audio signals in the FM system where pre-emphasis is not effective. The slope detection phenomenon causes the low frequency FM to be converted to an AM signal. The AM derived through slope detection of an FM signal thereafter will be detected in both channels thereby reducing the isolation between channels. Also, a true phase detector used to detect the PM component where pre-emphasis is effective will produce a nonlinear output where pre-emphasis is not effective. The principles of systems of this type are embodied in U.S. Pat. No. 3,068,475 and other references.